The invention described in the present application concerns a novel hydraulic valve device for controlling a double action servomotor. The servomotor is, in turn, adaptable for use in the automatic steering assembly of a self-propelled, agricultural vehicle set as a harvester which also includes a guidance system for sensing rows of plants, furrows, or similar guide references.
Known hydraulic valve assemblies of this type include four separate pressure line connections which are engageable in a predetermined manner in response to selective positioning of the hydraulic control valve into one of three control positions. The four separate pressure line connections provide a cumbersome valve device, which is relatively costly to manufacture and has an increased susceptibility to accidental hydraulic fluid leakage.
In comparison, the hydraulic device of the present invention eliminates these problems by including only two interconnected pressure line connections that may be engaged or disengaged responsive to the selective positioning of the hydraulic valve device.
Existing hydraulic valve devices often include either a piston or slide assembly to control the position of the servomotor. The piston is formed with a plurality of radial grooves arranged to direct the flow of hydraulic fluid through the valve in accordance with the predetermined control positions. Because the radial grooves function as control surfaces, it is necessary that the piston include a costly high quality surface finish. There must also be a certain amount of free play between the piston and its surrounding cylinder to avoid jamming as a result of any thermal expansion which might take place. Because of this, it is inevitable that there will be some seepage as the hydraulic fluid is forced past the piston, thereby reducing the volumetric effectiveness. Apart from this, the hydraulic fluid which has leaked through the valve assembly must be drained and when a piston is used in the hydraulic control device, special arrangements must be made to reduce any sudden pressure build-up of hydraulic fluid which might take place during the switching movement. As a result, the control edges of the piston are often notched to direct some of the hydraulic fluid from the pressure line connection into the return line connection during the switching movement.
It is evident that the effectiveness of such a piston assembly is thereby heavily dependent upon the purity of the hydraulic fluid. Foreign bodies in the hydraulic fluid, which in most cases is hydraulic oil, can quickly lead to "dragging" of the valve piston. This condition is especially disadvantageous for vehicle steering mechanisms, because the steering can be turned suddenly to its maximum angle without the possibility of rapid correction which increases the possibility of a dangerous accident.